In office equipment such as digital copiers and facsimile machines, original hard-copy documents are recorded as digital data using what can be generally called a “scanner.” In a typical scanner, a document sheet is illuminated and the light reflected from the document sheet is recorded by a photosensitive device such as a CCD or CMOS array, to be converted to digital image data. In one embodiment, a narrow strip of the document sheet is illuminated as the sheet is moved through a document handler, or the photosensitive device is moved relative to a platen on which the document sheet is placed.
Designing an illuminator for a scanner presents challenges in providing, among other aspects, an even illumination along the narrow strip of the document, as well as providing a suitable illumination profile across the narrow strip. Irregularities in the illumination level in the illuminated area will result in defects in the image data. An overview of the art of designing illuminators for scanners is given in U.S. Pat. No. 6,236,470.
One type of illuminator useful in document scanning includes a light-transmissive element that exploits internal reflections to direct light from one or more point sources to emerge in substantially parallel rays from an exit surface of the element toward a document. One known type of optical element, used in various contexts, includes a light-transmissive element that exploits internal reflections to direct light from one or more small sources to emerge in substantially parallel rays from an exit surface of the element. Specifically, one known shape for this purpose is a compound parabolic concentrator, or CPC; uses of the CPC are shown in U.S. Pat. No. 5,255,171 and US Patent Application Publication 2004/0131157.